Safety system for armament training and testing



Jn.u\1O,`11967* F. H. PREsTwooD 3,298,023

SAFETY SYSTEM FOR ARMAMENT TRAINING AND TESTING 2 Sheets-Sheet 1 Filed July 28,` 1965 Eig-1 70 rinus/w 1 f rnv G /PNTENA/ Y 6 '7 5?* 6 INVENTOR 1 SAFETY SYSTEM FOR ARMAMENT rRAlNrNG ,"Thednventon described herein may be manufactured andused by1orHfor the United States Government for I goyernmentalpurposes without payment to me of any 1 royalty thereon. Armament `trainingof interceptor pilotsV and the testying of: air-tofair missiles often require the use of radar augmented` `towed targets. 1 The tire control radar of the interceptor :iaircraft approaching ,the `towing plane and towstargettcombination acquires and locks on to a target whichllmay possibly be the towing aircraft. Since it is not always` p-ossible for the interceptor pilot to diiferentiatc` `betweenzrthetow target and the towing aircraft, due

to closure rate, the angle of attack or the lack of visibility, a `hazardous condition may exist. i awhighrdegree of range safety, it is most desirable that To maintain a idenite, unmistakable indication of interceptor radar acquisition and `lock-on of the `tow target exists, and that "the interceptor missiles are fail-safe disarmed until such `indication isiobtained and maintained. `;`It" sutherefore the primary purpose of this invention 1 top insure that `the interceptor radar directs the interceptoritire` `atthetow target` rather than at the towing airl craftzor other false target.

` tion are toprovideapparatus for accomplishing the above [purpose that is simple and reliable, that can be easily l added to existing interceptor radars and augmenters, that 3 is `completelyl automatic, ythat is usable over a wide fref 3 Vquency bandrwithout` adjustment and that is fail-safe in operation.`

Brieily, theseobjects are accomplished by providing "ongthe tow target `means for amplitude modulating the l output of the augmenter at a low frequency, for examl `ple` il() `c./s., and providing on the interceptor an arming i control circuit which receives the interceptor radar video i l signal `andenergizes the arming circuit only when the low Further objects of the invenfrequency modulation added at the `tow target has been present` onthe.; video signal` for a prescribed length of l@ time,` such as ve seconds.

Tlieinvention will be described in more `detail with reference to ,the specic embodiment thereof shown in i the accompanying drawingsin which ,1 FIG; H1 shows` a complete system` in block form incorporating `the invention.

FIG; i2 illustrates `schematically lthe apparatus added at the; tow target.

` 1 FIG. 1.3 `schematically represents the construction of the Variable RF attenuator at the tow target.

" FIGt-4 is a schematic `diagram of the arming circuit add'edqto the interceptor radar in accordance with the `invention.;`

` Referring toFlG; 1, block 1 represents a normal in- `terceptgor :dire control radar which periodically radiates short duration `pulsesmof high frequency energy from an- 1 tennaZ and.;receivesi echoes of these pulses through the sameantenna... Thewechoes are detected in the radar receiver to; produce video pulses which appear on video i Unlted States i Patent O output line l3.11 At `the tow target the radiated pulses are received .by antenna 4 and applied to augmenter 5 which `is a `radio `frequency amplifier that, for X-band radar, would;` normally be of the traveling wave type. Con- `ventionallyj theyamplied radio frequency pulses at the l `output :of the augmenter would be applied directly to .transmitting antenna 6 `for reradiation lto antenna 2 of ICC the interceptor radar. However, in accordance with the invention, a variable radio frequency energy attenuator 7 is interposed between the augmenter output and the transmitting antenna. The attenuation of this element is made to Vary at a low rate, such as l0 c./s., by modulator 8, thus placing a l() c./s. amplitude modulation on the pulses radiated from antenna 6. At the fighter aircraft, in -accordance with the invention, there is provided an arming control circuit 9 which receives the video pulse output of the radar receiver and energizes the arming circuit 10 only when a l0 c./s. modulation has been present on the video pulses for a prescribed interval, as already stated. Further protection against firing upon an undesired target is provided by the range tracking circuits of the radar 1 since, in order for the interceptors missiles to be armed, the video return having the proper amplitude modulated signature must be in time coincidence With the radar range gate.

The modulator 8 and the manner of coupling its output to the attenuato-r 7 are shown schematically in FIG. 2. The l0 c./s. modulating frequency is generated by a phase shift oscillator comprising transistor Q1 and a regenerative feedback circuit comprising elements C1, R1, C2, R2, R3, C3 and R5. Since the impedance of an RC circuit is capacitive, the voltage across each resistor leads the applied voltage. -Each RC section of the feedback circuit contributes approximately 60 degrees of phase advance at the frequency of oscillation, so that the output of Q1, adds in phase at the input to sustain oscillation. The oscillation frequency may be controlled by R2.

The nearly sinusoidal output of the oscillator at the collector of Q1 is applied through R8 to the base of emitter follower Q2. The emitter follower provides load isolation of the -oscillator for frequency stability. The output of the emitter follower is applied through C5 to the base of an overdriven amplifier Q3 which operates between cutoff and saturation to square the sinusoidal input. The squared output appears across R12 and the portion of it applied to the base of driver Q4 may be adjusted by this potentiometer. The parameters of the squaring circuit are such that an unsymmetrical square wave of approximately 2:1 time relation is produced. The emitter current of Q4 flows through current limiting resistor R15 Iand shielded conductor 12 to a solid state diode in attenuator 7 which acts as the RF attenuating element in a manner to be described later. In the ernbodiment shown, the voltage on conductor 12, which is the voltage applied across the diode, varies from O v. to +18 v. and causes a forward diode current of O to 50 ma.

The internal construction of the variable RF attenuator 7 is shown schematically in FIG. 3. The attenuator comprises a short section of rectangular waveguide 13, shown in longitudinal section, having couplings at each end to coaxial transmission lines 14 and 1S which serve to insert the attenuator between the output of augmenter 5 and transmitting antenna 6. The RF energy absorbing element in the attenuator is a PIN diode 16 connected for RF between opposite points inside the rectangular waveguide. As seen in FIG. 3, the cathode of the diode is connected directly to the waveguide wall, whereas an RF coupling to the waveguide for the anode may be established by a small annular capacitor 17 for which lead 12 serves as one element. As explained above, conductor 12 connects the anode of the diode 16 through R15 t-o the emitter of Q1. Therefore, Q4 controls the forward biasing voltage across this diode which, as stated above, varies from 0 v. to +18 v. in accordance with the rectangular output of this transistor.

The PIN diode 16 is a silicon device in which an intrnsic (I) layer is situated between P-type and N-type layers. It has the property that, at frequencies about 100 M c./s., the usual rectifying properties disappear and the diode behaves throughout the RF cycle as a linear resistance the value of which depends upon the forward biasing voltage. At zero bias the resistance of the diode is high relative to the characteristic impedance of the rectangular waveguide Iand the presence of the diode in the waveguide has little effect on the transmission of RF energy through it. However, as the forward bias is increased the resistance of the diode falls, causing an increasing portion of the RF energy to ow through the diode `where it is dissipated as heat due to the diode resistance. Thus, as the forward bias increases, the attenuation of attenuator 7 increases until it reaches, in the example given, a maximum value of about 20 db for a -forward bias of 18 volts. With this arrangement a rectangular amplitude modulation at l c./s. is impressed upon the RF pulses applied to transmitting antenna 6. Further information on PIN diodes may be found in the literature, for example, an article entitled The PIN Diode: A Versatile Microwave Component by Robert E. Heller appearing in the March 8, 1963, issue of Electronics, pages 40-43.

The schematic diagram of the arming circuit 9 is shown in FIG. 4. The video output of the interceptor radar 1, which consists of a series -of video pulses with a 10 c./s. more or less degraded rectangular amplitude modulation envelope resulting from the rectangular modulation of the RF pulses radiated by the tow target, is applied over line 3 to potentiometer R16. The first stage of the arming control circuit is an automatic gain control stage for compensating for the variations in video levels caused by varying ranges and angle of attack as the interceptor Iapproaches the target, and other parameters. The positive-going portions of the video signal are applied through CR1 to the base `of Q5 for amplification by this transistor, the ampliiied output at the collector being a 10 c./s. negative-going roughly rectangular Wave. This wave charges C9 through C8 and CR2 with the polarity shown. CR3 is a 5 volt Zener dio-de. When the sum of the positive input signal at the base of Q and the voltage of C9 exceeds 5 volts, diode CR3 breaks down and prevents the base potential of Q5 from rising appreciably iabove the value required to produce about 5 volts across C9' Thus the negative-going output of Q5 is limited to a maximum swing of approximately 5 volts. The minimum signal level that will cause limiting to occur is .adjustable by potentiometer R16.

The limited output of Q2 is applied cross sensitivity potentiometer R22 which adjusts the signal level required to energize the arming circuit. Thermistor R44 compensates this sensitivity adjustment for the effects of circuit parameter changes caused by temperature variations.

The negative-going pulses coupled to the base of Q6 through C19 are emitter coupled to Q7 and result in negative-going pulses at the collector of this transistor. Negative-going 10 c./s. pulses at the collector of Q7 excite the twin series-T active bandpass filter, composed of C12, C13, R29, R28, R30, C14 and C15, Causing It t0 at its inherent 10 c./s. frequency. Other frequency components appearing at the filter input are suppressed. A

10 c./s. sine wave is thus developed at the base of emit- 1 ter follower Q6 which provides the necessary high impendance load for the filter. Emitter output of Q8 is coupled to the base of differential amplifier Q1. The resulting collector current is therefore the differential between the 10 c./s. sine Wave and the input pulses. The sinusoidal output developed across R26 is coupled through C16 to the base of emitter follower Q9. The network comprised of C11, R45 and R16 stabilizes the D.C. operating point of Q7. Emitter follower Q9 provides isolation between the l0 c./s. lilter and the relay driver Q16.

In the quiescent state, Q19 is biased near cutoff` and C19 is charged to approximately 22 volts. Therefore only the positive half cycles of the 10 c./s. sine wave applied to the baseV of this transistor 'are effective.

Each positive half cycle causes Q19 to conduct heavily, energizing K1 and discharging C18. During the negative half cycles when Q15 is cut olf, C15 charges through K1 holding the relay in its `actuated state until the next positive half cycle occurs. Thus K1 remains energized as long as a 10 c./s. wave from the filter is present.

The active 10 c./s. ilter previously described is inherently a high Q device and has a tendency to ring at its center frequency when power is irst applied to the system or `when transients are passed through the radar receiver. An `arming delay circuit prevents these transients from energizing the arming circuit. This circuit comprises unijunction transistor Q11 and associated cornponents. In the quiescent condition (K1 deenergized), Q11 is not conducting and C19 is discharged through R39. When K1 energizes, R99 is removed from the circuit, +22 v. potential is lallplied to Q11 and C19 begins to charge through R15 toward +22 v. The time constant of R49 and C19 is approximately 3.6 seconds. After approximately 5 seconds, the voltage across C19 is sufficient to place Q11 in conduction and energize K2. Actuation of K2 energizes the arming circuit 10 through contact 20 and applies a holding voltage through contact 21 of K1, and contact 22 of K2 to the K2 winding so that K2 does not deenergize when C19 is discharged by conduction through Q11. Actuation of K2 also energizes arming indicator light 23 when the override switch S1 is in its out position as shown. When the l0 c./s. modulation is no longer present, K1 deenergizes removing power from the delay circuit and the holding voltage from K2 which releases, thus disarming the missile and extinguishing arming indicator 23. Also, R99 is shunted across C19 so that it is completely discharged in preparation for the next arming cycle.

The manual override switch S1 permits the pilot or maintenance personnel to override the norm-al safety arming procedure. An amber override indicator 24 illuminates when the override switch is actuated to its arming or in position,

I claim:

1. In an armament practice and testing arrangement in which a normally disarmed Weapon is directed at a practice target by radar apparatus including a radar receiver having la video output and in which the practice target carries a radar augmenter, safety apparatus forpreventing the arming of said weapon when directed by said radar apparatus toward -any target other than Isaid practice target, said safety apparatus comprising: means at said practice target for `amplitude modulating the output of said :augmenter ata predetermined low frequency; and frequency selective means located at said radar apparatus and receiving the video output therefrom for arming said weapon only after the continuous presence Afor a prescribed minimum time interval of an amplitu-de -modulation of said video output at said low frequency.

2. An armament practice and testing arrangement comprising: radar apparatus for directing a weapon at a practice target, said radar apparatus having a video output and said weapon having a normally deenergized electrical arming circuit; carried lby said practice target: a receiving antenna, a transmitting antenna, a radar augmenter and a variable attenuator for radio frequency energy, said augmenter being a radio frequency amplifier having its input coupled to the receiving antenna. and its output coupled through said attenuator to the transmitting antenna, and means coupled to said `attenuator for cyclically varying its attenuation at a predetermined low frequency; la very narrow band filter centered on said low frequency having its input coupled to the video output of ysaid radar apparatus; a resistor and a capacitor connected in series; a shunting resistor having a resistance value low relative to that of said series connected resistor; means coupled to the output of said filter for connecting said shunting resistor across said capacitor in the p absence of an output signal from said filter and for removingsaid `shunting resistor and applying a direct voltage; acrossrsaid series connected resistor and capacitor in the,` presence of an output` signal from said filter; and

means responsive to the voltage across said capacitor and operative `when said voltage exceeds a certain value to applylxan arming voltage to said arming circuit.

3.` Apparatus as claimed in claim 2 in which said variable" attenuator comprises a short section of rectangular `waveguide having -a coupling to the output of said augmenter at one end and a coupling to said transmitting antenna at the `other end and a PIN diode connected as an internal radio frequency shunt in said waveguide, and in `which said means for cyclically varying the attenuation ofisaid attenuator comprises means for applying an alternating bias voltage of said low frequency between the `anode and cathode of said PIN diode.

4. Anarrnament practice and testing arrangement comen having its inputcoupled to the receiving antenna and its;` outputzicoupledi through said attenuator to the transmitting antenna, and means coupled to said attenuator forclyclically varying its attenuation at a predetermined low frequency; a vary narrow band filter centered on said low frequency having its input coupled to the video output of said radar apparatus; a resistor and a capacitor connected in series; a unijunction transistor having said capacitor in its emitter circuit; an arming relay having its Winding connected in the output of said unijunction transistor, said relay having a rst pair of nOrmally open contacts connected between a source of arming voltage and said arming circuit and a second pair of normally open contacts connected in a holding circuit for said relay; a shunting resistor having a resistance value low relative to that of said series connected resistor; and means coupled to the output of said filter for connecting said shunting resistor across said capacitor in the absence of an output signal from said lter and for removing said shunting resistor and applying -a direct voltage across said series connected resistor and capacitor andato said unijunction transistor and to said holding circuit in the presence of an output signal from said lter.

References Cited by the Examiner UNITED STATES PATENTS 6/ 1949 Whitlock 3437 X 9/ 1965 Prestwood 343-7 X 

1. IN AN ARMAMENT PRACTICE AND TESTING ARRANGEMENT IN WHICH A NORMALLY DISARMED WEAPON IS DIRECTED AT A PRACTICE TARGET BY RADAR APPARATUS INCLUDING A RADAR RECEIVER HAVING A VIDEO OUTPUT AND IN WHICH THE PRACTICE TARGET CARRIES A RADAR AUGMENTER, SAFETY APPARATUS FOR PREVENTING THE ARMING OF SAID WEAPON WHEN DIRECTED BY SAID RADAR APPARATUS TOWARD ANY TARGET OTHER THAN SAID PRACTICE TARGET, SAID SAFETY APPARATUS COMPRISING: MEANS AT SAID PRACTICE TARGET FOR AMPLITUDE MODULATING THE OUTPUT OF SAID AUGMENTER AT A PREDETERMINED LOW FREQUENCY; AND FREQUENCY SELECTIVE MEANS LOCATED AT SAID RADAR APPARATUS AND RECEIVING THE VIDEO OUTPUT THEREFROM FOR ARMING SAID WEAPON ONLY AFTER THE CONTINUOUS PRESENCE FOR A PRESCRIBED MINIMUM TIME INTERVAL OF AN AMPLITUDE MODULATION OF SAID VIDEO OUTPUT AT SAID LOW FREQUENCY. 